Can you tell me

What difference does it make if you raise or lower blade height within the mower deck via moving spindle spacers . I have always put 4 spacers bottom and two on top giving 1" on bottom. Works fine for me but want to know if it would benefit anything (performance) to change them at all.

This time of year i cut pretty short because of all the leaves around here, if i keep the grass short the leaves roll right over to the neighbors yard ( and then they call me for a cleanup). But i keep 4 spacers on top and 2 on bottom and 4 washers on the blade. Obviously the more you use ( spacers and washers) the lower it will be.

In the spring when the grass is wet and thick I run the blades 1/4 inch above the bottom of the deck....this creates a larger tunnel for the grass clippings to flow and that means less clumping. I get the proper mowing height by adjusting the rear axle and the front castor wheels insted of the blades. If you have a low horsepower to blade ratio ex. 12.5 HP 48" cut then the deck should be pitched 1/4" lower in the front vs. the rear of the deck at this time so the blades don't have to double cut the grass. This helps to keep blade tip speed up. If you have lots of HP. then you can level the mowing deck pitch.

When spring has passed I raise the blades up to 1.5" above the bottom of the deck...with a level pitched deck. The unused 1.5" of deck drags along the grass and lays the grass in the direction of mowing. What this does is create a very nice striping pattern.
I leave the setup like this for the remainder of the season as it helps chop up the leaves (double gators) nicely and a lot less blowout.

Blade/spindle spacers allow more adjustability in blade height settings.
It depends on the inner height of the deck itself, the type blades being used, whether you're cutting/discharging or mulching.

Many mower manuals suggest a minimum number of spacers to be used between blade and spindle.

Don't really see how fewer than 3 spacers would work well. That would move the blades too close to the bottom of the deck surface and higher than the leading edge of the deck. The grass would have to be pretty stiff to pop up and get cut.

Probably very different depending on brand of mower and deck configuration.